1. Field of the Invention
The invention relates to a protective circuit for the overvoltage protection of a subscriber line interconnect circuit of a digital time multiplex-communication network, specifically, to a telephone network which includes an electronic interface circuit. More particularly, the invention relates to the overvoltage protection of the electronic switches associated with the subscriber line circuit for test accessibility, through which, each conductor of a core pair of the connected subscriber line circuit for each corresponding connection of the subscriber line interconnect circuit has provided an isolation of the separate conductors of the core pair from the connected subscriber line circuit. A separate test access connection to the conductors of the core pair and/or to the corresponding connections of the active subscriber line interconnect circuits is provided.
2. Description of the Prior Art
Protective circuits are necessary since the interface circuit of subscriber line interconnect circuits is comprised of electronic circuit elements, as are the circuits for test accessibility. Prior art subscriber line interconnect circuits, in which the interface circuit have repeating coils and voltage stable resistors, and in which test accessibility was accomplished by means of electro-magnetic relays, can withstand overvoltages in the order of lkV without damage, so that a protective circuit is redundant.
A protective circuit must meet the requirement that the resistance symmetry imposed upon the core pairs must remain unaffected. Furthermore there must be assurance that in the event of a current interruption resulting from a malfunction, it may be ascertained, at the subscriber interconnect circuit, whether this defect is a result of a subscriber line interconnect circuit itself, or of a defect in the protective circuit.